In response to the increasing market adoption of residential and commercial solar in the United States and elsewhere, and in order to address safety concerns for fire fighters and other first responders who may have to get on the roof of a home or other structure that has photovoltaic panels that are actively generating electricity, amendment 690.12 to the National Electric Code now requires a so-called “rapid shutdown function” that renders the system harmless within a matter of seconds. Specifically, the code amendment requires a rapid shutdown function that controls specific conductors as follows: (1) requirements for controlled conductors shall only apply to PV system conductors of more than 5 FT inside a building or 10 FT from a PV array; (2) controlled conductors shall be limited to not more than 30 volts and 240 volt-amperes within 10 seconds of shutdown; (3) voltage and power shall be measured between any two conductors and between any conductor and ground; (4) the rapid shut down methods shall be labeled as such: PHOTOVOLTAIC SYSTEM EQUIPPED WITH RAPID SHUTDOWN (in white, capitalized, reflective letters on a red background); and equipment that performs the rapid shutdown shall be listed and identified. 2014 NEC 690.12.
In residential systems, the effect of this code requirement is that system installers/operators must provide a mechanism by which the conductive lines that supply the power produced by the PV array to the inverter (hereinafter referred to as “power lines”) are disconnected from the PV array so that the power lines pose no risk of electric shock.
Typically, inverters will contain a DC power shutoff switch that prevents power generated by the PV array from flowing to the inverter. However, some jurisdictions may require a separate DC shutoff switch between the positive/negative PV power lines (that extend between the PV array and the inverter) and the inverter which, in the event of an emergency, could be shut off. Such a shutoff may be located in a separate box near an inverter. In still further jurisdictions, separate shutoff may be unnecessary because a first responder may simply pull the AC power meter out of the meter socket, or throw a breaker switch at or near the meter socket so that no electricity can flow into the system, thereby also shutting down any PV system inverters. Despite these existing shutoff mechanisms which either shut down the inverter or break the circuit between the inverter and the panels, there is still a need to rapidly cutoff flow of power from the array (so-called rapid shutdown), and in some cases there is also a need to discharge to a safe level any residual charge that remains on the positive/negative PV power lines.
There are existing implementations of the rapid shutdown function, but they suffer from various limitations that make them undesirable. For example, electrically controlled switches such as DC contactors or relays are in some cases used to effect rapid shutdown, however, these devices are relatively expensive, large in size, and prone to failure, making them unappealing for large scale implementation, in particular in an industry whose goal is to reduce the cost per kilowatt in order to be competitive with fossil-fuel-based energy.